In an LC (liquid chromatography)--MS (mass spectrometry) system, the effluent emerging from a liquid chromatography is directly admitted into a mass spectrometer. In this system, the effluent from the liquid chromatography comprises solvent and mobile phase contained glycerol, which functioned as the matrix. A known ion source for use in this kind of mass spectrometer is shown in FIG. 1.
Referring to FIG. 1, the mass analyzer of the mass spectrometer has a housing 1. Another housing 2 constitutes an ion source. The inside of each housing is connected with several vacuum pumps (not shown) via an exhaust pipe 3 or other pipes (not shown) to maintain it as a high vacuum. A partition wall 4 separates the mass analyzer from the ion source, and is centrally provided with a hole 4a. In the housing 2, four insulating poles 5 surrounding the hole 4a stand on the surface of the partition wall 4, so that a chamber block 6 is fixed to the partition wall 4. The space 7 neighboring the surface of the chamber block 6 is called the ionization chamber. Also, the chamber block 6 itself is often called the ionization chamber. A sample holder 8 is inserted in a hole 6a formed at the center of the chamber block 6 so as to plug up the hole 6a. The holder 8 is mounted at the front end of a cylindrical sample inlet member 9 which extends into the atmosphere through the housing 2 of an ion source. A thin inlet tube 10 extends through the inlet member 9 along its axis. The end of the tube 10 which is in the atmosphere is connected with a micro high-performance liquid chromatograph 11. The other end of the tube 10 that is in the evacuated housing is close to the end surface of the sample holder 8. A particle beam generator 12 for producing a beam of particles, such as neutral particles or ions, is mounted in a hole formed in the upper portion of the housing 2 of an ion source. The beam 13 produced by the generator 12 bombards (irradiates) effluent which exudes from the end surface of the sample holder 8. As a result, the effluent is ionized. The produced ions are extracted in the direction indicated by the arrow A by the action of an electric field set up by the sample holder 8 and converging and accelerating electrodes 14 supported by the insulating poles 5. Some of the ring-shaped electrodes 14 are notched for not blocking the path of the beam 13. The holder 8 is maintained at a given electric potential. Then, the ions are analyzed according to their mass-to-charge ratio in the mass analyzer.
FIG. 2 is a cross-sectional view of the prior art structure of the sample holder 8 shown in FIG. 1. As an example, the inlet tube 10 which introduces effluent from the side of the atmosphere is made of fused silica. A tube 16 made of stainless steel is mounted around the inlet tube 10. A glass tube 17 is mounted around the tube 16. A porous member 18 obstructs the opening of the inlet tube 10. As an example, the porous member consists of a filter made of frit that is fabricated by sintering powdered stainless steel. The porous member 18 is shaped into a disk, and has a thickness of 0.2 mm and a diameter of about 1 mm. The porous member 18 is bonded to the glass tube 17 via adhesive 19 which is applied not only to the contact surface of the porous member 18 with the glass tube 17 but to the side surface of the porous member 18 as shown to prevent effluent from leaking (exuding) off through the side surface of the porous member 18. A repeller power supply 20 applies an appropriate voltage to the porous member 18 via the stainless steel tube 16 to set up an electric field within the ionization chamber 7, for converging the ions emanating from the surface of the porous member 18 and accelerating them in the direction indicated by the arrow A (FIG. 1).
In the conventional structure constructed as described above, the effluent emerging from the liquid chromatograph 11 is admitted into the ionization chamber 7 via the inlet tube 10. The effluent passes through the porous member 18 at the front end of the inlet tube 10 and exudes from the surface of the porous member 18. Then, the effluent is bombarded with a particle beam 13 produced by the particle beam generator 12. The resulting ions are introduced into the mass analyzer 1 where they are analyzed. This conventional instrument is disclosed in the Journal of Chromatography, 346 (1985) 161-166, Elsevier Science Publishers B.V., Amsterdam.
In the above-described conventional instrument, the effluent from the liquid chromatograph is directly admitted into the ion source and forced through the porous member to ionize the effluent. In this case, it is required that all the effluent transported be ionized and that previously conveyed sample components do not linger. In order to fulfill these requirements, the flow rate of the effluent into the ion source, the amount of effluent consumed per unit time in the ion source must be balanced. The amount of effluent consumed per unit time in the ion source is determined by the amount of evaporation of the effluent, especially the solvent accounting for a large proportion of it, in a vacuum per unit time, and the amount of ionization and sputtering of the effluent per unit time due to bombardment of the particle beam. Considering the capability of the existing ion source, the adequate flow rate is less than 1 .mu.l/min. However, the flow rate of the effluent from the micro high-performance liquid chromatograph is as much as about 1 ml/min. to 100 .mu.l/min. Therefore, an appropriate splitter must be used to reduce the flow rate approximately by two or three orders of magnitude.
However, it is quite difficult for the existing splitter to achieve such small amounts of flow rate. Therefore, it is inevitable that the flow rate is so large as to unbalance the aforementioned factors. This permits unionized effluent to build up on the surface of the porous member 18 under the influence of gravity as indicated by numeral 21. This buildup 21 is also bombarded with the beam 13 and ionized. Therefore, sample components which are separated one after another by high-performance liquid chromatograph gather at the buildup 21 and are ionized simultaneously and analyzed in the mass analyzer. This phenomenon, known as memory effect, yields undesirable results.